The Grand Duke responded to the spiritual power once again. However, the "response" was far from as clear as that of the animals and lacked any substantive meaning. It was merely an instinctual acknowledgment, akin to a person hearing someone call their name and absentmindedly saying "oh".
Even the lowest intelligence fish or invertebrates such as earthworms communicated with spiritual power in a manner far more complex than the Grand Duke.
The Grand Duke before them did not seem to be "communicating" as much as it was similar to a person shouting into a valley and receiving some echoes in return.
But regardless of how weak the response was, any response was still a response. Spiritual power was not effective on all forms of "life" but was limited to animals. It had absolutely no effect on plants.